There is a need for processes that can capture CO2 from the flue or fuel gases generated by e.g. power stations, cement and steel factories, in order to mitigate global warming from the CO2 emissions from such installations. The separation of CO2 from gases produced in industrial processes, such as flue gas, is the first step in a Carbon Capture and Sequestration (CCS) process in which the CO2 is separated, compressed to a high density fluid, transported and sequestered in deep saline aquifers, depleted oil and gas fields, deep coal seams, or deep ocean reservoirs.
Because of the very large investment associated with industrial infrastructure that emits CO2, it is preferable that the separation process can be retrofitted to capture the flue gases emitted from such existing infrastructure, in a process called Post Combustion Capture (PCC). There are many existing chemical and physical processes for separating CO2 from flue or fuel gases, but the barriers to widespread adoption of any such process are technical, economic and environmental. For power plants, a viable PCC CCS process is typically expected to meet the following specifications:                The cost of CO2 separation, including compression to the high density fluid, is less than US$20 tonne−1; and        The efficiency of capture of CO2 from the flue gas exceeds 90%; and        The efficiency of production of electricity (Electrical Power Output to Lower Heat Value of the Fuel) is reduced by not less than 5%; and        There is no additional environmental harm.        
There is currently no CCS process that can meet all of these specifications. The cost of separation is recognised to be the highest cost of all the CCS process steps. An amine based technology, called MEA, was developed for separating CO2 from gases for enhanced oil and gas recovery. However, amines in general, and ammonia, produce toxic wastes, and the large environmental footprint may be such that amine and ammonia based processes may not be acceptable for large scale CCS, even if the other technoeconomic barriers to adoption can be met.
In addition to PCC, the CO2 can be removed in the industrial process as a pure CO2 gas stream. For example, the combustion of fossil fuels with pure oxygen leads to a gas which is essentially pure CO2, for CCS. However, the cost of separating oxygen from air is an expensive process that fails to meet the specification for the cost of CO2 separation. In another approach, fossil fuels are processed with steam in gasifiers to produce a fuel gas comprising hydrogen, CO2 and CO, and the CO is converted to CO2 by a water-gas shift reactor. The CO2 is separated from the hydrogen. The amine approach can be used to scrub the CO2 from these fuel gas streams, as well as other solvents such as alcohols. There are energy penalties, and environmental impacts, in the use of these scrubbing processes. Also, a CCS process for removing CO2 from fuel gas streams at higher temperatures may be derived.
A promising approach for the CCS separation technology is the Calcium Regenerable Sorbent (CRS) process, which is also called CaO Looping. Heesink et al (A. B. M. Heesink and H. M. G. Temmink “Process for removing carbon dioxide regeneratively from gas streams” U.S. Pat. No. 5,520,984) first described this process in 1994 for removing CO2 from flue gas and fuel gas streams. The system they describe involves a carboniser in which the sorbent, CaO or MgO.CaO, reacts with the CO2 in the gas stream to produce CaCO3 or MgO.CaCO3. It also describes transfer of the reacted sorbent to a second reactor, the calciner, in which the CO2 is released as a pure gas stream and the sorbent is regenerated and looped back into the carboniser. The energy to drive this process is applied to the calciner and is released from the carboniser through a heat exchanger where it can be used to generate energy. Shimizu et al. (T. Shimizu, T. Hirama, H. Hosoda, K. Kitano, “A twin bed reactor for removal of CO2 from combustion processes”, Trans I Chem E, 77A, 1999) proposed that CaO carbonation at 600° C. could be used to capture CO2 from the flue gas to produce CaCO3, and regeneration/calcination of CaO above 950° C. would occur by burning fuel with the CaCO3, akin to conventional calcination, but with pure oxygen from a separating plant, so as to give pure CO2 and steam as an output. Limestone, CaCO3, is used as the feedstock.
The CRS approach has been further studied by MacKenzie et al (A. MacKenzie, D. L. Granatstein, E. J. Anthony and J. C. Abanades, “Economics of CO2 Capture using the Calcium Cycle with a Pressurized Fluidized Bed Combustor” Energy Fuels 2007, 21 (2), 920-926.). They demonstrated that such a system could meet the specification for CCS set out above. However, the thermal energy needed to drive the calcination system requires that the oxyfuel plant consumes about ⅔ as much fuel as used by the power plant. In systems in which the temperature of the calciner is higher than the temperature of the carboniser, as described in the above-mentioned prior art, the loss of efficiency of the sorbent by sintering requires a higher flow of sorbent through the reactors and a higher demand for energy, and a larger plant.
Abanades Garcia et al (Abanades Garcia, J Carlos and J. Oakey; “Combustion method with integrated CO2 separation by means of carbonation” US Patent publication no. 20050060985) describe a CRS process in which the thermal energy is extracted from a combustor as an additional element to the carboniser and calciner. They claim a system based on a fluid circulating fluid bed reactor, drawn bed reactor, or cyclone reactor. Their patent discloses that heat transfer from a combustion reactor provides heat to drive the calciner, and the use of a partial vacuum or steam in the operation of the calciner. They specify a calcination temperature of 900° C. and a carbonation temperature between 600-750° C., with the operating temperature of the combustor exceeding that of the calciner.
Albanades et al (“Capturing CO2 from combustive gases with a carbonation calcination loop. Experimental results in a 30 kW test facility”, 2a Reunion de Seccion Espagnol del Instituto de Combustion, 8-9 May, 2008) describe a system in which the heat transfer from the combustor reactor can be facilitated by the cycling of the CaO sorbent between the combustor, calciner, and the carboniser, again in a configuration where the combustor temperature is larger than the calciner temperature, which in turn is larger than the carboniser temperature. This reduces the requirement for heat transfer through the walls of the reactors, which can be difficult to achieve in practice. However, the high temperature in the combustor leads to an additional high degree of sorbent sintering, which further reduces the efficiency of the sorbent to capture the CO2.
A characteristic of the schemes referenced above is that the heat flows from the hot calciner to the cooler carboniser, and it follows that the energy burden of the CCS plant will necessarily be large, because the enthalpy of calcination is a large fraction of the heat of combustion per mole of CO2. Also, as mentioned above, the efficiency of the sorbent to capture CO2 is severely reduced due to sorbent sintering. The sorbent can be replenished by fresh limestone, but this introduces another energy penalty from the calcination of the limestone to the sorbent. The specifications for CCS can thus typically only be met by using the large amount of heat produced by the carboniser unit to drive turbines to produce electricity. In effect, this CRS process must typically be integrated into the power plant electricity generation system to meet the specifications, but even then the size and cost of the plant is significant. The extraction of heat from the plant combustor draws energy away that would otherwise be used to generate power, and releases heat at the lower temperature of the carbonator. Even if the heat is transformed to power, there is a decrease of the efficiency of the plant to produce power.
Limestone can be used as the feedstock in the above described prior art. The advantage of using limestone as the feedstock to produce the lime sorbent is that limestone is already used in power plants for Flue Gas Desulphurisation (FGD), and the spent sorbent from the CO2 separation system can be used for FGD. Thus no additional mined limestone may be required and the waste product from FGD is gypsum, which is already accounted for in the environmental and economic impact of the power plants. Thus the CCS specification of “no additional environmental harm” with respect to the inputs and outputs of the CRS process can be met when CRS is combined with FGD. However, if the sorbent sinters and has to be refreshed by injection of fresh limestone, this advantage can be lost because the consumption of limestone for CRS exceeds the need for lime for FGD. Sorbents other than CaO can be used, such as MgO, K2O and Na2O and MgO.CaO in these metal oxide looping systems.
Thus the existing CCS systems based on CRS may satisfy the technoeconomic and environmental specifications for CCS, but are characterised by demanding a high throughput of energy to separate the CO2. To meet the CCS specifications, such systems must burn significant fuel e.g. in a separate combustor from that of the power plant, or extract heat from the combustion system of that plant. This either increases the capital costs and the footprint of a CCS system or reduces the efficiency of the power plant and the CO2 produced leads to a decrease of the net CO2 avoided. This impact applies to other carbon capture technologies that rely on sorbents that have a high CO2 binding energy.
A need therefore exists to provide a system and method for removing CO2 from a gas stream that seek to address at least one of the above mentioned problems.